1. Field of the Invention
This invention relates to an optical mirror which can be suitably used for lightweight large mirrors, high-speed revolving polygonal mirrors, high output laser reflecting mirrors, copier drums, movable mirrors for cameras and X-ray mirrors.
2. Description of the Prior Art
At present, large-size mirrors, high-speed revolving polygonal mirrors and high output laser reflecting mirrors are constituted by ultrahigh precision finishing of aluminum alloy or oxygen-free copper through turning, milling, etc., to a predetermined shape and then forming a mirror surface by coating with gold or the like.
Desirably, these mirrors should not be deformed by heat of the light they reflect or by centrifugal or other externally applied forces. However, the material used for forming the mirror surface of the mirror has a lower specific gravity and coefficient of thermal expansion and a higher modulus of elasticity than those of the metallic material used as substrate. Therefore, deformation of the mirror under gravitational and centrifugal forces, and heat etc. is inevitable. Further, deformation of the material from which the mirror surface is formed caused at the time of the machining sometimes appears as a strain with temperature increase at the time of use or over long use.